Dry Ingredients: In a medium bowl, whisk together bread flour, all-purpose flour, cocoa powder, baking soda, and salt.
Melt Chocolate: Microwave butter and chopped chocolate in 30-second intervals until smooth. Let cool for 5 minutes.
Mix Wet: In a large bowl, combine vegetable oil with sugars, then add the melted chocolate mixture and beat until combined.
Add Eggs: Add eggs one at a time, beating well, then stir in vanilla.
Chill Dough: Gradually mix dry ingredients into wet. Cover and refrigerate for at least 2 hours.
Prep & Shape: Preheat oven to 350°F (175°C). Roll 1.5 tablespoons of dough into balls, place on sheets, and flatten with your palm.
Pattern: Create a crosshatch pattern on top with a fork.
Bake: Bake for 8-10 minutes. Allow to cool completely on the baking sheets.
Make Filling: Beat butter until fluffy, add powdered sugar and cocoa, then vanilla, salt, and heavy cream until smooth.
Assemble: Spread 2 tablespoons of filling on one cookie and top with another. Wrap in plastic and chill for 2 hours.